1. Field of the Invention
The present invention relates to a mobile radio receiver for receiving a signal in a desired frequency band and, more particularly, to a receiver having a circuit extracting accurately an RSSI signal indicative of the strength of a received radio wave.
2. Description of the Related Art
In portable transceivers as a terminal of mobile communications, such as an automobile radio apparatus and portable telephone, a high carrier frequency (in GHz) assigned to its radio system is modulated with a number of frequency-divided subcarriers, and each subcarrier frequency is then modulated with voice data, video data and control data at a channel within a particular frequency band out of tens of channels, each of which is divided in time slots to perform transmission and reception.
In such a mobile radio system, the input received signal level received at the terminal greatly varies depending on operational environments, and an assumption must be made that the received signal level varies in excess of 80 dB because of the distance between a terminal and a base station and level variations due to fading.
Also contemplated are adverse operational environments in which a signal level at an intended channel is low while a strong interfering wave is present at an adjacent channel.
In the transceiver system of such radio mobile system, a high-frequency circuit of superheterodyne reception system is employed. As a filter for extracting accurately a desired frequency, a filter employing a bulk device (such as surface acoustic wave (SAW) filter or piezoelectric device) is used to satisfy the requirements of a high sharpness and a wide dynamic range.
Since the transceiver system, as a portable terminal, must be as compact and light-weight in design as possible, the use of an active filter constructed of an IC circuit between intermediate frequency stages is contemplated.
FIG. 1 shows one example of a high-frequency receiving circuit of a terminal. A radio wave transmitted by a base station is received by an antenna 100. The received signal is amplified by a high-frequency amplifier 101 constructed of a low-noise device such as a GaAs FET device, and is mixed with a signal a local oscillator 103 supplies to a first mixer 102. The received signal is thus converted into a first intermediate frequency. A predetermined band is extracted from the first converted intermediate frequency by a SAW filter 104 or the like, is amplified by an intermediate frequency amplifier 105, and is fed to a second mixer 106.
The second mixer 106 is supplied with a signal from a second local oscillator 107 for selecting a desired channel frequency, and feeds a signal in a frequency band of a desired channel as a second intermediate frequency to a variable gain circuit 108.
A filter 109 (active filter) is provided to select the signal in the second intermediate frequency band.
The signal within the desired frequency band extracted by the active filter 109 is wave-shaped by a multistage limiter 111 to have a constant amplitude, and is then input to a demodulated-signal processing circuit 120 for data extraction.
The multistage limiter 111 has a gain of 20-30 dB per stage, and has a gain of 100 dB if it is a four-stage one, for example.
As a result, in normal reception conditions, the limiter 111 operates to output a signal in its substantially saturated state regardless of variations in the level of the received signal, and the output signal is clipped in a rectangular wave at a constant signal level and is fed to the demodulated-signal processing circuit 120.
In such a receiver, each gain stage 111(a, b, c, d) of the limiter 111 becomes saturated starting from a later one as a signal level rises. The output levels of the gain stages 111(b, b, c, d) of four are respectively detected by detectors 112(a, b, c, d). The output of these detectors are summed by an adder 113, and the summed signal is then smoothed by a low-pass filter 114 or the like. An RSSI (Receiving Signal Strength Indicator) output signal is thus obtained.
As in an invention already proposed by the inventors of this invention, the use of the active filter 109 as a filter to select a channel within a desired frequency band results in a narrow dynamic range compared with an ordinary solid filter element. For this reason, the variable gain circuit 108 is placed in front of the active filter 109 so that the output of a level detector 110 controls the variable gain circuit 108 to provide the gain control characteristics shown in FIG. 2A.
By controlling the input level to the filter to input a signal appropriately set in view of the dynamic range of the active filter 109, the filter is controlled not to be saturated over a wide received range and the extraction operation of an accurate channel frequency is thus permitted.
If the gain control is carried out to prevent the active filter 109 from saturating, the signal level fed to subsequent stages of the filter becomes different from that originally intended. In its normal operation, the limiter 111 must operate to saturate at any received signal levels, but if the signal level is controlled by the variable gain circuit 108 as shown in FIG. 2A, the signal level fed to the limiter 111 is limited at a signal level point P where the variable gain circuit 108 starts its control operation. The linearly increasing trend of the RSSI output indicative of the signal level is degraded as shown by a dotted line R in FIG. 2B, and the desired requirements of the receiving system, for example, the accurate switching of diversity and accurate detection operation of idle channels, are not satisfied.